Field of the Invention
The present invention relates to a document reading device which reads an image of a document to be conveyed, an image processing device equipped with the document reading device, a document reading method, and a document reading program.
Description of the Related Art
When a document is fed by an automatic document conveying device and is read by a reading device, an inclination (also referred to as skew) occurs with respect to an ideal reading position, due to an inclination at the time of setting the document or a mechanical conveyance accuracy.
As a method of correcting the skew, a method of physically correcting the skew by causing a document to abut against resist rollers has been conventionally known. In this method, however, because the document is made to abut against the resist rollers, a collision noise of the sheet occurs.
Therefore, in recent years, in order to reduce the noise, especially in a multifunctional digital image forming device called a multifunction peripheral (MFP) or the like, techniques of performing the skew correction in the image processing are being adopted. As one of them, there is a technique in which a background plate is disposed in a direction traversing the conveyance direction of the document on the background of the reading position, reflected light when the document passes through the background plate is received by a reading unit such as a CCD line sensor, and the inclination of the document is detected based on the image data output from the reading unit to correct the image data of the document in accordance with the detected inclination (for example, JP 2013-110505 A).
However, conventionally, when the skew correction is performed by the image processing, because the same sensor reads the background plate and the document image at the time of passage of the document for the skew correction, the following problems has occurred.
That is, when the skew correction is performed by the image processing, it is necessary to accurately detect a boundary (a leading end of the document) of the document from the image data that is output from the reading unit. However, if the background plate remains white, it is not possible to accurately perform gradation separation from shadow generated between the background plate and the documents. For this reason, although it is conceivable to hold a gradation in the background plate (for example, to make the ground plate black), there are problems such as an occurrence of ground fog or deterioration of set-off when the document is thin sheet. Furthermore, there are problems in image quality such as punched holes or ear folds in the document, or blackening of the margin when the size is irregular.
As a countermeasure to these problems, by providing a reflecting member of a mirror surface material on the background plate in an inclined manner to make a regular reflected light of the illumination light incident on the reading unit, the reading image of the document background is set to be equal to or higher than a reading upper limit (a saturation value) to secure contrast, and detection of presence or absence of document dependence is partially achieved.
However, although the aforementioned problems can be solved in this method, this method requires strict maintenance and management of the installation precision of the background plate (mirror), resulting in another problem of a significant increase in cost.
Incidentally, recently, in order to accurately reproduce colors for color images, while reading monochrome images at high speed, there is a tendency of increases in a document reading devices in which a monochrome reading unit including a Gy (gray) line sensor for reading a monochrome image is added to a color reading unit made up of three line sensors for R (red), G (green) and B (blue) for reading the color images.
Even in the document reading device having the color reading unit and the monochrome reading unit, when trying to detect and correct the inclination of the document by providing the background plate, because the same problems as the above-mentioned problem arise, the correspondence measures are required.